Semiconductor device including a floating gate

ABSTRACT

A semiconductor device includes a semiconductor layer with a trench dug downward from its surface, a source region formed on a surface layer portion adjacent to a first side of the trench in a prescribed direction, a drain region formed on the surface layer portion, adjacent to a second side of the trench opposite to the first side in the prescribed direction, a first insulating film on the bottom surface and the side surface of the trench, a floating gate stacked on the first insulating film and opposed to the bottom surface and the side surface of the trench through the first insulating film, a second insulating film formed on the floating gate, and a control gate at least partially embedded in the trench so that the portion embedded in the trench is opposed to the floating gate through the second insulating film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 13/525,406, filedJun. 18, 2012, which was a division of U.S. application Ser. No.12/470,439, filed May 21, 2009. These prior US applications claimed thebenefit of priority of Japanese application 2008-135514, filed May 23,2008, and the present continuation application likewise claims thebenefit of priority of Japanese application 2008-135514. The disclosuresof these prior US and foreign applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including afloating gate nonvolatile storage element.

2. Description of Related Art

An EEPROM (Electrically Erasable Programmable Read Only Memory) is knownas a typical nonvolatile memory.

FIG. 13 is a schematic plan view of a conventional EEPROM. FIG. 14 is aschematic sectional view of the EEPROM taken along a line XIII-XIII inFIG. 13.

An EEPROM 121 includes a plurality of memory cells arrayed in the formof a matrix in a direction X and a direction Y orthogonal thereto. Eachmemory cell includes an N-type first diffusion region, an N-type seconddiffusion region 124 and an N-type third diffusion region 125 formed inthe surface layer portion of a P-type silicon substrate 122 at intervalsin the direction X. A first insulating film is laminated on the siliconsubstrate 122. Each memory cell further includes a floating gate 126 anda select gate 127 formed on the first insulating film. The floating gate126 is formed to extend over the first diffusion region 123 and thesecond diffusion region 124 in plan view. A control gate 129 is providedon the floating gate 126 through a second insulating film 128. Thecontrol gate 129 is formed to cover the upper surface and the sidesurfaces of the floating gate 126. On a position where the seconddiffusion region 124 and the floating gate 126 are opposed to eachother, the first insulating film is partially removed, and then a tunnelwindow (a tunnel insulating film) 130 generally rectangular in plan viewis formed at the removed portion. The tunnel window 130 is thinner thanthe first insulating film. On the other hand, the select gate 127 isformed to extend over the second diffusion region 124 and the thirddiffusion region 125 in plan view.

Thus, each memory cell has a memory transistor consisting of the firstdiffusion region 123, the second diffusion region 124, the firstinsulating film, the floating gate 126, the second insulating film 128and the control gate 129. Further, each memory cell has a selecttransistor consisting of the second diffusion region 124, the thirddiffusion region 125, the first insulating film and the select gate 127.

A bit line 131 extending in the direction X is provided above thecontrol gate 129 through an interlayer dielectric film. The bit line 131is connected to the third diffusion regions 125 (drain regions of theselect transistors) of the memory cells arrayed in the direction X underthe same through contact plugs 132. The control gates 129 of the memorycells arrayed in the direction Y are integrated into a word lineextending in the direction Y. The select gates 127 of the memory cellsarrayed in the direction Y are integrated into a select line extendingin the direction Y. The first diffusion regions 123 (source regions ofthe memory transistors) of the memory cells arrayed in the direction Yare integrated into a source line extending in the direction Y.

Referring to FIGS. 13 and 14, illustration of the first insulating filmand the interlayer dielectric film is omitted.

As shown in FIG. 13, each pair of memory cells adjacent to each other inthe direction X have symmetrical structures with respect to a straightline extending therebetween in the direction Y. The first diffusionregion 123 is shared by the memory cells provided on both sides of thefirst diffusion region 123 in the direction X as the source regions ofthe memory transistors. Thus, the cell size (the area of each memorycell) is reduced.

However, further increase in capacity and downsizing are required to anonvolatile memory such as an EEPROM, and the cell size must be furtherreduced in order to satisfy the requirements.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor deviceallowing further reduction of a cell size.

A semiconductor device according to an aspect of the present inventionincludes: a semiconductor layer; a trench dug downward from the surfaceof the semiconductor layer; a source region formed on the surface layerportion of the semiconductor layer adjacently to a first side of thetrench in a prescribed direction; a drain region formed on the surfacelayer portion of the semiconductor layer adjacently to a second side ofthe trench opposite to the first side in the prescribed direction; afirst insulating film formed on the bottom surface and the side surfaceof the trench; a floating gate stacked on the first insulating film andopposed to the bottom surface and the side surface of the trench throughthe first insulating film; a second insulating film formed on thefloating gate; and a control gate at least partially embedded in thetrench so that the portion embedded in the trench is opposed to thefloating gate through the second insulating film.

In the semiconductor device, the trench is dug in the semiconductorlayer downward from the surface thereof. The source region and the drainregion are formed on the surface layer portion of the semiconductorlayer. The source region is formed on the first side of the trench inthe prescribed direction, and adjacent to the trench. The drain regionis formed on the side of the trench opposite to the source region in theprescribed direction, and adjacent to the trench. The first insulatingfilm is formed on the bottom surface and the side surface of the trench.The floating gate is provided on the first insulating film. The floatinggate is opposed to the bottom surface and the side surface of the trenchthrough the first insulating film. The second insulating film is formedon the floating gate. The control gate is provided on the secondinsulating film. The control gate is at least partially embedded in thetrench. The portion of the control gate embedded in the trench isopposed to the floating gate through the second insulating film.

The source region, the drain region, the first insulating film, thefloating gate, the second insulating film and the control gateconstitute a nonvolatile storage element (a floating gate memorytransistor). The first insulating film is in contact with the drainregion on the side surface of the trench. The floating gate is opposedto the drain region through a portion of the first insulating film incontact with the drain region. Therefore, charges stored in the floatinggate FN (Fowler-Nordheim)-tunnel through the portion of the firstinsulating film in contact with the drain region. In other words, atunnel window (a tunnel insulating film) is arranged on the side surfaceof the trench in the nonvolatile storage element provided on thesemiconductor device. Thus, no planar space is required for the tunnelwindow. Therefore, the cell size can be reduced by at least the space ascompared with the structure (see FIG. 14) having the tunnel windowopposed to the surface of the drain region.

In the structure having the tunnel window opposed to the surface of thedrain region, the drain region is arranged under the floating gate.Therefore, the drain region is formed in advance of the formation of thefloating gate. More specifically, the drain region is formed by forminga resist pattern selectively exposing a portion defining the drainregion on the semiconductor layer and doping the surface layer portionof the semiconductor layer with an impurity through the resist patternserving as a mask in advance of the formation of the floating gate. Inthe structure having the tunnel window opposed to the surface of thedrain region, further, the tunnel window is formed by partially reducingthe thickness of the insulating film provided on the semiconductorlayer. In order to partially reduce the thickness of the insulatingfilm, a resist pattern selectively exposing the portion to be reduced inthickness is formed on the insulating film, and the insulating film isetched through the resist pattern serving as a mask.

In the structure according to the present invention, on the other hand,the source region and the drain region are arranged on side portions ofthe floating gate, and hence the source region and the drain region canbe formed by doping the whole area of an active region (a region exposedfrom an element isolation region) of the semiconductor layer with animpurity. Therefore, no resist patterns are required for forming thesource region and the drain region. Further, the portion of the firstinsulating film in contact with the drain region forms the tunnelwindow, whereby the portion of the first insulating film forming thetunnel window may not be selectively etched, and no resist pattern isrequired therefor. Therefore, the number of reticles necessary formanufacturing the semiconductor device can be reduced. Consequently, thenumber of manufacturing steps and the manufacturing cost can be reduced.

Preferably, the first insulating film has a thin portion, having arelatively small thickness, in contact with the drain region and a thickportion, having a relatively large thickness, formed by the remainingportion of the first insulating film other than the thin portion.

FN tunneling of charges can be excellently caused due to the formationof the thin portion. On the other hand, the capacitance between thefloating gate and the semiconductor layer can be reduced due to theformation of the thick portion, whereby the coupling ratio (the ratio ofthe capacitance between the floating gate and the control gate to thesum of the capacitance between the floating gate and the control gateand the capacitance between the floating gate and the semiconductorlayer) can be improved.

More preferably, the thick portion has a first thick portion formed onthe side surface of an opening-side end portion of the trench andcontinuous with the thin portion and a second thick portion formed on aside opposite to the first thick portion through the thin portion andcontinuous with the thin portion.

The first thick portion is formed on the side surface of theopening-side end portion of the trench, whereby the thin portion and thefirst thick portion are in contact with the drain region. Thus, the sizeof the thin portion causing FN tunneling is reduced, whereby undesiredcharge escape from the floating gate can be suppressed. Further, thesize of the thick portion is enlarged, whereby the coupling ratio can befurther improved.

The control gate may be shaped to be inside the second insulating filmas viewed from the depth direction of the trench.

The floating gate, the second insulating film and the control gate mayprotrude beyond the upper end of the trench. In this case, thesemiconductor device may further include a third insulating filmlaminated on the semiconductor layer, and portions of the floating gate,the second insulating film and the control gate protruding beyond theupper end of the trench may be covered with the third insulating film.

The semiconductor device may further include a third insulating filmlaminated on the semiconductor layer, and the upper ends of the floatinggate, the second insulating film and the control gate may be flush withthe surface of the third insulating film.

The control gate may be shaped to integrally have a body portionarranged on the trench to protrude from the trench and an extendingportion extending sideward from the body portion.

The extending portion is opposed to the floating gate in the depthdirection of the trench. When the control gate has the extendingportion, therefore, the capacitance between the floating gate and thecontrol gate can be increased, and the coupling ratio can be furtherimproved.

When the control gate has the extending portion, the semiconductordevice may further include a third insulating film laminated on thesemiconductor layer, and the upper end portion of the second insulatingfilm may extend onto the floating gate and may be arranged on the thirdinsulating film, while the extending portion may be arranged on thethird insulating film through the second insulating film.

The foregoing and other objects, features and effects of the presentinvention will become more apparent from the following detaileddescription of the embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a semiconductor device according to afirst embodiment of the present invention.

FIG. 2 is a schematic sectional view of the semiconductor device takenalong a line II-II in FIG. 1.

FIGS. 3A to 3O are schematic sectional views for illustrating a methodof manufacturing a memory cell of the semiconductor device shown in FIG.2 in step order.

FIG. 4 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention.

FIGS. 5A to 5D are schematic sectional views for illustrating a methodof manufacturing a memory cell of the semiconductor device shown in FIG.4 in step order.

FIG. 6 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention.

FIGS. 7A to 7C are schematic sectional views for illustrating anothermethod of manufacturing a memory cell of the semiconductor device shownin FIG. 6 in step order.

FIG. 8 is a schematic sectional view of a semiconductor device accordingto a forth embodiment of the present invention.

FIGS. 9A to 9C are schematic sectional views for illustrating a methodof manufacturing a memory cell of the semiconductor device shown in FIG.8 in step order.

FIG. 10 is a schematic sectional view of a semiconductor deviceaccording to a fifth embodiment of the present invention.

FIGS. 11A to 11O are schematic sectional views for illustrating a methodof manufacturing a memory cell of the semiconductor device shown in FIG.9 in step order.

FIG. 12 is a schematic sectional view of a semiconductor deviceaccording to a sixth embodiment of the present invention.

FIG. 13 is a schematic plan view of a conventional EEPROM (semiconductordevice).

FIG. 14 is a schematic sectional view of the EEPROM taken along a lineXIII-XIII in FIG. 13.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are now described with reference tothe attached drawings.

FIG. 1 is a schematic plan view of a semiconductor device according to afirst embodiment of the present invention. FIG. 2 is a schematicsectional view of the semiconductor device taken along a line II-II inFIG. 1.

A semiconductor device 1 is an EEPROM constituted of a plurality ofmemory cells arrayed in the form of a matrix in a direction X and adirection Y orthogonal thereto.

The semiconductor device 1 includes a P-type semiconductor layer 2 madeof silicon as a substrate thereof.

A plurality of first diffusion regions 3 are formed on the surface layerportion of the semiconductor layer 2 at regular intervals in thedirection X. Each first diffusion region 3 is an N-type diffusion regioncontaining an N-type impurity diffused therein, and linearly extends inthe direction Y.

On both sides of each first diffusion region 3 in the direction X, aplurality of memory cell regions are alignedly set in the direction Y.Each memory cell region is surrounded by a field oxide film 4 formed onthe surface layer portion of the semiconductor layer 2 in a U shape (a Ushape having an opening end arranged on the side of the first diffusionregion 3 in plan view) opened toward the first diffusion region 3 inplan view, and isolated from memory cell regions adjacent thereto in thedirection X.

In each memory cell region, a trench 5 generally tetragonal in plan viewis dug in the semiconductor layer 2 downward from the surface thereof.One side surface of the trench 5 in the direction X is in contact withthe first diffusion region 3.

In each memory cell region, further, an N-type second diffusion region 6and an N-type third diffusion region 7 are formed on the surface layerportion of the semiconductor layer 2. The second diffusion region 6 isformed on a side of the trench 5 opposite to the first diffusion region3, and in contact with another side surface of the trench 5 in thedirection X. The third diffusion region 7 is formed on a side of thesecond diffusion region 6 opposite to the trench 5 at an interval fromthe second diffusion region 6.

A first insulating film 8 made of silicon oxide is formed on the bottomsurface and the side surfaces of the trench 5. The first insulating film8 is so formed that the thickness of a portion in contact with the firstdiffusion region 3 and the second diffusion region 6 is relatively smalland the thickness of the remaining portion (a portion in contact withthe P-type region of the semiconductor layer 2) is relatively large. Inother words, the first insulating film 8 integrally includes a thinportion 9, having a relatively small thickness, in contact with thefirst diffusion region 3 and the second diffusion region 6 and a thickportion 10, having a relatively large thickness, consisting of theremaining portion other than the thin portion 9.

A floating gate 11 made of doped polysilicon (polysilicon doped with anN-type impurity in a high concentration, for example) is formed on thefirst insulating film 8. The floating gate 11 is opposed to the bottomsurface and the side surfaces of the trench 5 through the firstinsulating film 8.

A second insulating film 12 having an ONO (oxide film-nitride film-oxidefilm) structure where a silicon nitride film is sandwiched between apair of silicon oxide films, for example, is formed on the floating gate11 to cover the overall inner surface of the floating gate 11.

A control gate 13 made of doped polysilicon is provided on the secondinsulating film 12, to fill up the inner side of the second insulatingfilm 12.

The upper end portions of the floating gate 11, the second insulatingfilm 12 and the control gate 13 protrude upward from the trench 5. Thefloating gate 11 and the second insulating film 12 protrude from theupper end of the trench 5 (the surface of the semiconductor layer 2)with quantities generally identical to each other, while the controlgate 13 protrudes from the trench 5 with a quantity larger than thesame. Outside the trench 5, a step is formed between an end surface (theupper surface) of the second insulating film 12 and the surface (theupper surface) of the control gate 13 due to the difference between thequantities of protrusion.

A third insulating film 14 made of silicon oxide is laminated on thesemiconductor layer 2. The third insulating film 14 covers the surfaceof an active region (a region not provided with the field oxide film 4)of the semiconductor layer 2 and the portions of the floating gate 11,the second insulating film 12 and the control gate 13 protruding fromthe trench 5.

On the third insulating film 14, a select gate 15 made of dopedpolysilicon is formed on a position opposed to the space between thesecond diffusion region 6 and the third diffusion region 7. The selectgate 15 overlaps with the second diffusion region 6 and the thirddiffusion region 7 in the direction X in plan view. In other words, bothend portions of the select gate 15 in the direction X are opposed to thesecond diffusion region 6 and the third diffusion region 7 respectively.A sidewall 16 made of silicon nitride or silicon oxide is formed aroundthe select gate 15.

FIG. 1 omits illustration of the third insulating film 14 and thesidewall 16, in order to facilitate understanding of the structure ofthe semiconductor device 1.

The first diffusion region 3 as well as the second diffusion region 6,the first insulating film 8, the floating gate 11, the second insulatingfilm 12 and the control gate 13 provided on each memory cell regionconstitute a memory transistor 17 of each memory cell. The firstdiffusion region 3 and the second diffusion region 6 function as asource region and a drain region of the memory transistor 17respectively. The second diffusion region 6, the third diffusion region7, the third insulating film 14 and the select gate 15 provided on eachmemory cell region constitute a select transistor 18 of each memorycell. The second diffusion region 6 and the third diffusion region 7function as a source region and a drain region of the select transistor18 respectively. In other words, each memory cell includes one memorytransistor 17 and one select transistor 18.

Two memory cells formed on the first diffusion region 3 and two memorycells on both sides thereof in the direction X have symmetricalstructures with respect to a straight line (extending in the directionY) set therebetween along the first diffusion region 3, and share thefirst diffusion region 3 as the source regions of the memory transistors17.

An interlayer dielectric film (not shown) made of silicon oxide islaminated on the third insulating film 14, the select gate 15 and thesidewall 16. A word line 19 is provided on the interlayer dielectricfilm. The word line 19 extends in the direction Y to be opposed to thecontrol gates 13 of the memory cells arrayed in the direction Y, and iselectrically connected with all control gates 13 opposed thereto throughcontact plugs 20. More specifically, contact holes (not shown) facingthe control gates 13 respectively are penetratingly formed in the thirdinsulating film 14 and the interlayer dielectric film. The contact plugs20 made of tungsten are embedded in the contact holes respectively. Theword line 19 made of aluminum is provided on the interlayer dielectricfilm to couple the top faces of the contact plugs 20 arrayed in thedirection Y with one another. Thus, the word line 19 is connected incommon to the control gates 13 of the memory cells arrayed in thedirection Y through the contact plugs 20.

A first-layer bit line 21 is further provided on the interlayerdielectric film separately from the word line 19, to extend over twomemory cells adjacent to each other in the direction X through the fieldoxide film 4. Both end portions of the first-layer bit line 21 areconnected to the third diffusion regions 7 (the drain regions of theselect transistors) of the two memory cells through contact plugs 22.More specifically, contact holes (not shown) facing the third diffusionregions 7 are penetratingly formed in the third insulating film 14 andthe interlayer dielectric film. The contact plugs 22 made of tungstenare embedded in the contact holes respectively. The first-layer bit line21 is provided on the interlayer dielectric film so that both endportions thereof are opposed to the two third diffusion regions 7adjacent to each other in the direction X through the field oxide film4. Thus, the first-layer bit line 21 is connected in common to the thirddiffusion regions 7 of the two memory cells adjacent to each other inthe direction X through the field oxide film 4, through the contactplugs 22.

The word line 19 and the first-layer bit line 21 are covered with asecond interlayer dielectric film (not shown) made of silicon oxide. Asecond-layer bit line 23 made of aluminum is provided on the secondinterlayer dielectric film. The second-layer bit line 23 extends in thedirection Y, and is opposed to first bit lines 21 arranged on the samestraight line in the direction Y. The second-layer bit line 23 iselectrically connected with all first-layer bit lines 23 opposed theretothrough vias 24 made of tungsten. Thus, the second-layer bit line 23 isconnected in common to the third diffusion regions 7 of the memory cellsarrayed in the direction Y through the contact plugs 22, the first-layerbit line 21 and the contact plugs 22.

The select gates 15 of the memory cells arrayed in the direction Y areintegrated into a select line extending in the direction Y.

In order to write data in the memory transistor 17 (the memory cell),the first diffusion region 3 (the source region of the memory transistor17) and the third diffusion region 7 (the second-layer bit line 23) areset to the ground potential (0 V) respectively. Further, a prescribedvoltage Vpp (10 to 12 V, for example) is applied to the control gate 13(the word line 19) and the select gate 15. Thus, the select transistor18 is turned on, and a high electric field is formed between the seconddiffusion region 6 (the drain region of the memory transistor 17) andthe control gate 13. When the high electric field is formed, electronsare injected from the second diffusion region 6 into the floating gate11 by FN tunneling through the first insulating film 8, to achieve datawriting.

In order to erase the data, on the other hand, the first diffusionregion 3 (the source region of the memory transistor 17) is opened, andthe control gate 13 is set to the ground potential. Further, aprescribed erasing voltage (the voltage Vpp, for example) is applied tothe select gate 15 and the third diffusion region 7 (the drain region ofthe select transistor 18). Thus, the select transistor 18 is turned on,a high voltage is applied to the second diffusion region 6 (the drainregion of the memory transistor 17), and a high electric field is formedbetween the second diffusion region 6 and the floating gate 11. When thehigh electric field is formed, electrons are extracted from the floatinggate 11 to the second diffusion region 6, to achieve data erasing.

A threshold voltage for allowing the source region and the drain reignof the memory transistor 17 to conduct (a voltage to be applied to thecontrol gate 13 for allowing the source region and the drain region toconduct) varies with a state where electrons are stored in the floatinggate 11 and a state where no electrons are stored therein. In otherwords, the threshold voltage is at a relatively high level Vth(1) whenthe floating gate 11 stores electrons, and at a relatively low levelVth(0) when the floating gate 11 stores no electrons.

In data reading from the memory transistor 17, a prescribed gate voltageand a prescribed drain voltage are applied to the select gate 15 and thethird diffusion region 7 (the drain region of the select transistor 18)respectively, and whereby the select transistor 18 is turned on. Thefirst diffusion region 3 (the source region of the memory transistor 17)is set to the ground potential, and a sense voltage Vsense of anintermediate level between the voltages Vth(1) and Vth(0) is applied tothe control gate 13. When a current flows between the source region andthe drain region of the memory transistor 17 due to the application ofthe sense voltage Vsense, a logic signal “0” can be obtained. If nocurrent flows between the source region and the drain region of thememory transistor 17 due to the application of the sense voltage Vsense,on the other hand, a logic signal “1” can be obtained.

As hereinabove described, the first insulating film 8 is in contact withthe second diffusion region 6 serving as the drain region of the memorytransistor 17 on the side surface of the trench 5. The floating gate 11is opposed to the second diffusion region 6 through the portion of thefirst insulating film 8 in contact with the second diffusion region 6.Therefore, electrons stored in the floating gate 11 FN-tunnel throughthe portion of the first insulating film 8 in contact with the seconddiffusion region 6. In other words, a tunnel window (a tunnel insulatingfilm) is arranged on the side surface of the trench 5 in the memorytransistor 17. Therefore, no planar space is required for the tunnelwindow. Thus, the cell size can be reduced by at least the space ascompared with the structure shown in FIGS. 13 and 14, i.e., thestructure having the tunnel window 130 opposed to the surface of thesecond diffusion region 124 serving as the drain region of the memorytransistor.

In the left memory cell shown in FIG. 13, it is assumed that A denotesthe interval between the left edge of the control gate 129 and the leftedge of the floating gate 126 and the interval between the right edge ofthe floating gate 126 and the right edge of the control gate 129. It isalso assumed that B denotes the interval between the left edge of thefloating gate 126 and the left edge of the tunnel window 130, C denotesthe interval between the right edge of the tunnel window 130 and theright edge of the second diffusion region 124, D denotes the intervalbetween the right edge of the second diffusion region 124 and the leftedge of the first diffusion region 123 and E denotes the intervalbetween the left edge of the first diffusion region 123 and the rightedge of the floating gate 126 respectively. It is also assumed that Fdenotes a design rule, i.e., the size of the tunnel window 130 in thedirection X. In this case, the width W of the control gate 129 (the wordline) in the direction X is equal to 2A+B+C+D+E+F. If A=0.375 μm,B=0.375 μm, C=0.375 μm, D=0.75 μm, E=0.3 μm and F=0.6 μm, for example,the width W is equal to 3.15 μm.

In the left memory cell shown in FIG. 1, on the other hand, it isassumed that G denotes the interval between the left edge of the wordline 19 and the left edge of the contact plug 20 and the intervalbetween the right edge of the contact plug 20 and the right edge of theword line 19. It is also assumed that the size of the contact plug 20 inthe direction X is the design rule F. In this case, the width W of theword line 19 in the direction X is equal to 2G+F. If G=0.4 μm and F=0.6μm, for example, the width W is equal to 1.4 μm. Thus, in the structureshown in FIG. 1, the width W of the word line 19 can be reduced ascompared with the structure shown in FIG. 13, whereby the size of thememory cell in the direction X can be reduced.

The first insulating film 8 integrally has the thin portion 9, havingthe relatively small thickness, in contact with the first diffusionregion 3 and the second diffusion region 6 and the thick portion 10,having the relatively large thickness, consisting of the remainingportion other than the thin portion 9. FN tunneling of electrons can beexcellently caused due to the formation of the thin portion 9. On theother hand, the capacitance between the floating gate 11 and thesemiconductor layer 22 can be reduced due to the formation of the thickportion 10, whereby a coupling ratio R (the ratio of the capacitancebetween the floating gate 11 and the control gate 13 to the sum of thecapacitance between the floating gate 11 and the control gate 13 and thecapacitance between the floating gate 11 and the semiconductor layer 2)can be improved.

If the depth of the trench 5 (the distance from the surface of thesemiconductor layer 2 to the bottom surface of the trench 5) is 2.0 μm,the width of the trench 5 in the direction X is 1.4 μm, the depths ofthe first diffusion region 3 and the second diffusion region 6 are 0.2μm, the thickness of the thin portion 9 is 0.012 μm, the thickness ofthe thick portion 10 is 0.1 μm, the thickness of the floating gate 11 is0.1 μm and the thickness of the second insulating film 12 is 0.02 μm inthe structure shown in FIG. 2, for example, the coupling ratio R isexpressed as follows:R={(1.8+1.8+1.0)/0.02}/[{0.2+0.2)/0.012}+{(1.7+1.7+1.2)/0.1}+{(1.8+1.8+1.0)/0.02}]≈0.74Thus, data can be written in and erased from the memory transistor 17(the memory cell) with the voltage Vpp of a low practical level.

FIGS. 3A to 3O are schematic sectional views showing a method ofmanufacturing each memory cell in step order.

In the process of manufacturing each memory cell of the semiconductordevice 1, a sacrificial oxide film made of silicon oxide is first formedon the surface of a P-type silicon substrate 31 by thermal oxidation, asshown in FIG. 3A. Then, a silicon nitride film is formed on thesacrificial oxide film by LPCVD (Low Pressure Chemical VaporDeposition). The sacrificial oxide film and the silicon nitride film arepatterned, to form a hard mask 32 having an opening in a portion opposedto a portion for forming the trench 5.

Thereafter the silicon substrate 31 is etched through the hard mask 32,as shown in FIG. 3B. Thus, a trench 33 is formed in the siliconsubstrate 31.

Then, thermal oxidation is performed while leaving the hard mask 32 onthe silicon substrate 31, thereby forming the thick portion 10 made ofsilicon oxide on the inner surface of the trench 33. After the formationof the thick portion 10, the hard mask 32 is removed with phosphoricacid and hydrofluoric acid, as shown in FIG. 3C. Slight film loss iscaused in the thick portion 10 when hydrofluoric acid is employed (whenthe sacrificial oxide film is removed).

Thereafter a P-type epitaxial layer 34 made of silicon is formed byepitaxy, as shown in FIG. 3D. The epitaxial layer 34 is not formed onthe thick portion 10, but selectively formed only on the surface of thesilicon substrate 31.

Then, an oxide film 35 is formed on the surface of the epitaxial layer34 by thermal oxidation, as shown in FIG. 3E. The epitaxial layer 34 andthe silicon substrate 31 provided under the same constitute the P-typesemiconductor layer 2. The trench 33 partially forms the trench 5.

Thereafter a doped polysilicon film 36 is formed on the thick portion 10and the oxide film 35 by LPCVD, as shown in FIG. 3F.

The doped polysilicon film 36 is partially removed by CMP (ChemicalMechanical Polishing) until the oxide film 35 is exposed, as shown inFIG. 3G. Consequently, the doped polysilicon film 36 remains on thetrench 5, and the remaining doped polysilicon film 36 forms the floatinggate 11.

Then, a silicon oxide film, a silicon nitride film and a silicon oxidefilm are successively stacked on the floating gate 11 and the oxide film35 by oxidation and CVD, as shown in FIG. 3H. Thus, an ONO film 37 isformed on the floating gate 11 and the oxide film 35.

Then, doped polysilicon 38 is deposited on the ONO film 37 by LPCVD, asshown in FIG. 3I. The deposition of the doped polysilicon 38 iscontinued until the doped polysilicon 38 has a proper thickness on theONO film 37 outside the trench 5.

The doped polysilicon 38 is planarized and partially removed by CMP oretch-back until the ONO film 37 is exposed, as shown in FIG. 3J.Consequently, the doped polysilicon 38 remains on the trench 5, and theremaining doped polysilicon 38 forms the control gate 13.

Thereafter the oxide film 35 and the ONO film 37 are partially removedfrom on a portion of the semiconductor layer 2 located outside thetrench 5 with hydrofluoric acid or the like to be left only on thetrench 5, as shown in FIG. 3K. The oxide film 35 left on the trench 5forms the thin portion 9. The ONO film 37 left on the trench 5 forms thesecond insulating film 12.

Then, the third insulating film 14 is formed on the semiconductor layer2, the thin portion 9, the floating gate 11, the second insulating film12 and the control gate 13 by thermal oxidation to collectively coverthe same, as shown in FIG. 3L. Further, a doped polysilicon film 39 isformed on the third insulating film 14 by LPCVD.

Thereafter the doped polysilicon film 39 is selectively removed(patterned) by photolithography and etching, as shown in FIG. 3M. Thus,the select gate 15 is formed.

After the formation of the select gate 15, a silicon nitride film isformed on the third insulating film 14 by LPCVD. The silicon nitridefilm is formed to have a thickness for burying the select gate 15therein. The silicon nitride film is left only around the select gate 15by etch-back to form the sidewall 16, as shown in FIG. 3N.

Thereafter an N-type impurity (arsenic ions, for example) ision-implanted into the overall active region of the semiconductor layer2 from the surface thereof. A heat treatment is performed for diffusingthe N-type impurity, thereby forming the first diffusion region 3, thesecond diffusion region 6 and the third diffusion region 7 on thesurface layer portion of the semiconductor layer 2, as shown in FIG. 3O.Thus, the memory cell of the semiconductor device 1 is obtained.

The thin portion 9 of the first insulating film 8 forms the tunnelwindow, and hence the portion of the first insulating film 8 forming thetunnel window may not be selectively etched, and no resist pattern isrequired therefor. In the semiconductor device 1, therefore, the numberof reticles necessary for manufacturing the same can be reduced ascompared with the semiconductor device (the EEPROM 121) having thestructure shown in FIGS. 13 and 14. Consequently, the number of themanufacturing steps and the manufacturing cost can be reduced.

FIG. 4 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention. Referring to FIG. 4,portions corresponding to those shown in FIG. 2 are denoted by the samereference numerals respectively. In relation to the structure shown inFIG. 4, the following description is made with reference to pointsdifferent from those of the structure shown in FIG. 2, and redundantdescription is omitted as to the portions corresponding to those shownin FIG. 2.

In a semiconductor device 41 shown in FIG. 4, a third insulating film 14covers only the surface of an active region of a semiconductor layer 2.

A second insulating film 42 having an ONO structure is formed on afloating gate 11. The upper end portion of the second insulating film 42extends onto the floating gate 11, and is arranged on the thirdinsulating film 14.

A control gate 43 made of doped polysilicon is provided on the secondinsulating film 42. The control gate 43 is generally T-shaped insection, fills up a recess formed on a trench 5 by the second insulatingfilm 42, and has a prescribed thickness on the upper end portion of thesecond insulating film 42. In other words, the control gate 43integrally has a body portion 44 arranged on the trench 5 so that theupper end portion protrudes upward from the trench 5 and an extendingportion 45 extending sideward from the upper end portion of the bodyportion 44.

The extending portion 45 is opposed to the floating gate 11 in the depthdirection of the trench 5. Therefore, the capacitance between thefloating gate 11 and the control gate 43 can be increased due to theextending portion 45 provided on the control gate 43, and a couplingratio R can be further improved.

In the semiconductor device 41, a word line extending in a direction Ycan be formed by forming the upper end portion of the control gate 43(the upper end portion of the body portion 44 and the extending portion45) to extend in the direction Y and integrating the upper end portionsof control gates 43 of memory cells arrayed in the direction Y. Thus, noword line 19 shown in FIG. 2 is required. Therefore, an interlayerdielectric film (not shown) is laminated on the third insulating film 14and a bit line 46 extending in a direction X is provided on theinterlayer dielectric film, and the bit line 46 can be connected incommon to third diffusion regions 7 of the memory cells arrayed in thedirection X under the bit line 46 through contact plugs 47. As comparedwith the semiconductor device 1 shown in FIG. 2, therefore, the numberof wiring layers can be reduced by one, and the thickness of thesemiconductor device 41 can be reduced.

FIGS. 5A to 5D are schematic sectional views for illustrating a methodof manufacturing each memory cell of the semiconductor device 41 shownin FIG. 4.

The steps of manufacturing the semiconductor device 41 shown in FIG. 4partially overlap with the steps of manufacturing the semiconductordevice 1 shown in FIG. 1. In other words, the steps shown in FIGS. 3A to3I are first successively carried out, in order to manufacture thesemiconductor device 41.

After doped polysilicon 38 is deposited on an ONO film 37, the dopedpolysilicon 38 is selectively removed (patterned) by photolithographyand etching to form the control gate 43, as shown in FIG. 5A.

Then, a portion of the ONO film 37 exposed from the control gate 43 anda portion of an oxide film 35 located under the same are removed by RIE(Reactive Ion Etching) or the like, as shown in FIG. 5B. The oxide film35 and the ONO film 37 left unremoved form a thin portion 9 an thesecond insulating film 42 respectively.

Thereafter the third insulating film 14 is formed on the semiconductorlayer 2 by thermal oxidation, as shown in FIG. 5C.

Then, a doped polysilicon film is formed on the third insulating film 14by LPCVD, and the doped polysilicon film is selectively removed(patterned) by photolithography and etching. Thus, a select gate 15 isformed, as shown in FIG. 5D.

After the formation of the select gate 15, the steps shown in FIGS. 3Nand 3O are successively carried out, to obtain the memory cell of thesemiconductor device 41.

FIG. 6 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention. Referring to FIG. 6,portions corresponding to those shown in FIG. 4 are denoted by the samereference numerals respectively. In relation to the structure shown inFIG. 6, the following description is made with reference to pointsdifferent from those of the structure shown in FIG. 4, and redundantdescription is omitted as to the portions corresponding to those shownin FIG. 4.

In a semiconductor device 63 shown in FIG. 6, an upper end portion of afloating gate 11 is arranged on a third insulating film 14 at aperipheral portion of a trench 5. The portion of the floating gate 11 onthe peripheral portion is sandwiched between the third insulating film14 and a second insulating film 42.

FIGS. 7A to 7C are schematic sectional views for illustrating anothermethod of manufacturing each memory cell of the semiconductor device 63shown in FIG. 6.

According to the method, the steps shown in FIGS. 3A to 3F are firstsuccessively carried out.

After formation of a doped polysilicon film 36, an ONO film 61 having anONO structure is laminated on the doped polysilicon film 36 by oxidationand CVD, as shown in FIG. 7A.

Then, doped polysilicon 62 is deposited on the ONO film 61 by LPCVD, asshown in FIG. 7B. The deposition of the doped polysilicon 62 iscontinued until the doped polysilicon 62 has a proper thickness on theONO film 61 outside the trench 5.

Then, the doped polysilicon 62 is selectively removed (patterned) byphotolithography and etching to form the control gate 43, as shown inFIG. 7C. A portion of the ONO film 61 exposed from the control gate 43is removed by RIE or the like. Further, a portion of the dopedpolysilicon film 36 exposed through the removal of the ONO film 61 and aportion of the oxide film 35 located under the same are removed withhydrofluoric acid or the like. The oxide film 35, the doped polysiliconfilm 36 and the ONO film 61 left unremoved form the thin portion 9, thefloating gate 11 and the second insulating film 42 respectively.

Thereafter the steps shown in FIGS. 5D and 3N and 3O are successivelycarried out, to obtain the memory cell of the semiconductor device 63.

FIG. 8 is a schematic sectional view of a semiconductor device accordingto a forth embodiment of the present invention. Referring to FIG. 8,portions corresponding to those shown in FIG. 2 are denoted by the samereference numerals respectively. In relation to the structure shown inFIG. 8, the following description is made with reference to pointsdifferent from those of the structure shown in FIG. 2, and redundantdescription is omitted as to the portions corresponding to those shownin FIG. 2.

In the semiconductor device 1 shown in FIG. 2, the upper end portions ofthe floating gate 11, the second insulating film 12 and the control gate13 protrude upward from the trench 5, and are covered with the thirdinsulating film 14.

In a semiconductor device 71 shown in FIG. 8, on the other hand, a thirdinsulating film 14 covers only the surface of an active region of asemiconductor layer 2. The upper surfaces of a floating gate 11, asecond insulating film 12 and a control gate 13 are generally flush withthe surface of the third insulating film 14.

Also according to the structure of the semiconductor device 71, effectssimilar to those of the semiconductor device 1 shown in FIG. 2 can beattained.

FIGS. 9A to 9C are schematic sectional views for illustrating a methodof manufacturing each memory cell of the semiconductor device 71 shownin FIG. 8.

The steps of manufacturing the semiconductor device 71 shown in FIG. 8partially overlap with the steps of manufacturing the semiconductordevice 1 shown in FIG. 2. In other words, the steps shown in FIGS. 3A to3F are first successively carried out, in order to manufacture thesemiconductor device 71.

After formation of a doped polysilicon film 36, an ONO film 81 having anONO structure is laminated on the doped polysilicon film 36 by oxidationand CVD, as shown in FIG. 9A.

Then, doped polysilicon 82 is deposited on the ONO film 81 by LPCVD, asshown in FIG. 9B. The deposition of the doped polysilicon 82 iscontinued until the doped polysilicon 82 has a proper thickness on theONO film 81 outside a trench 5.

Thereafter the doped polysilicon 82, the ONO film 81 and the dopedpolysilicon film 36 are successively partially removed by CMP oretch-back until an oxide film 35 is exposed, as shown in FIG. 9C. Whenthe oxide film 35 is exposed, the doped polysilicon 82, the ONO film 81and the doped polysilicon film 36 remain only on the trench 5, to formthe control gate 13, the second insulating film 12 and the floating gate11 respectively.

Thereafter the oxide film 35 is removed from a portion of thesemiconductor layer 2 located outside the trench 5 with hydrofluoricacid or the like, so that the oxide film 35 left on the side surfaces ofthe trench 5 form a thin portion 9. Then, the steps shown in FIGS. 3L to3O are successively carried out, to obtain the memory cell of thesemiconductor device 71.

FIG. 10 is a schematic sectional view of a semiconductor deviceaccording to a fifth embodiment of the present invention. Referring toFIG. 10, portions corresponding to those shown in FIG. 2 are denoted bythe same reference numerals respectively. In relation to the structureshown in FIG. 10, the following description is made with reference topoints different from those of the structure shown in FIG. 2, andredundant description is omitted as to the portions corresponding tothose shown in FIG. 2.

In a semiconductor device 91 shown in FIG. 10, the thickness of a firstinsulating film 8 on the bottom surface of a trench 5 is small ascompared with that in the semiconductor device 1 shown in FIG. 1. Thedifference in thickness results from difference between a manufacturingmethod described below and the manufacturing method shown in FIGS. 3A to3O.

FIGS. 11A to 11O are schematic sectional views for illustrating a methodof manufacturing each memory cell of the semiconductor device 91 shownin FIG. 10.

In the process of manufacturing the memory cell of the semiconductordevice 91 shown in FIG. 10, a sacrificial oxide film made of siliconoxide is first formed on the surface of a semiconductor layer 2 bythermal oxidation, as shown in FIG. 11A. Then, a silicon nitride film isformed on the sacrificial oxide film by LPCVD. Then, a hard mask 101having an opening in a portion opposed to a portion for forming thetrench 5 is formed by patterning the sacrificial oxide film and thesilicon nitride film.

Thereafter a silicon substrate 31 is etched through the hard mask 101,as shown in FIG. 11B. Thus, the trench 5 is formed in the siliconsubstrate 31. After the formation of the trench 5, the hard mask 101 isremoved with phosphoric acid and hydrofluoric acid.

Then, an oxide film 102 is formed on the overall surface of the siliconsubstrate 31 including the inner surface of the trench 5 by thermaloxidation, as shown in FIG. 11C.

Thereafter the oxide film 102 is removed from the bottom surface of thetrench 5, the side surfaces of an opening-side end portion thereof and aportion of the silicon substrate 31 located outside the trench 5 byetch-back and washing so that the oxide film 102 is left only on theside surfaces of the trench 5, as shown in FIG. 11D.

Then, an oxide film 103 is formed on the bottom surface of the trench 5and the surface of the portion of the silicon substrate 31 locatedoutside the trench 5 by thermal oxidation, as shown in FIG. 11E. Theoxide film 102 left on the side surfaces of the trench 5 and the oxidefilm 103 left on the bottom surface of the trench 5 are integrated intoa thick portion 10.

Thereafter a doped polysilicon film 104 is formed on the thick portion10 and the oxide film 103 by LPCVD, as shown in FIG. 11F.

The doped polysilicon film 104 is partially removed by CMP until theoxide film 103 is exposed, as shown in FIG. 11G. Consequently, the dopedpolysilicon film 104 remains on the trench 5, and the remaining dopedpolysilicon film 104 forms the floating gate 11.

Then, an ONO film 105 having an ONO structure is formed on the floatinggate 11 and the oxide film 103 by CVD, as shown in FIG. 11H.

Then, doped polysilicon 106 is deposited on the ONO film 105 by LPCVD,as shown in FIG. 11I. The deposition of the doped polysilicon 106 iscontinued until the doped polysilicon 106 has a proper thickness on theONO film 105 outside the trench 5.

The doped polysilicon 106 is planarized and partially removed by CMP oretch-back until the ONO film 105 is exposed, as shown in FIG. M.Consequently, the doped polysilicon 106 remains on the trench 5, and theremaining doped polysilicon 106 forms the control gate 13.

Thereafter the oxide film 103 and the ONO film 105 are removed from aportion of the semiconductor layer 2 located outside the trench 5 withhydrofluoric acid or the like so that the oxide film 103 and the ONOfilm 105 are left only on the trench 5, as shown in FIG. 11K. The oxidefilm 103 left on the trench 5 forms the thin portion 9. The ONO film 105left on the trench 5 forms the second insulating film 12.

Then, a third insulating film 14 is formed on the semiconductor layer 2,the thin portion 9, the floating gate 11, the second insulating film 12and the control gate 13 by thermal oxidation to collectively cover thesame, as shown in FIG. 11L. Further, a doped polysilicon film 107 isformed on the third insulating film 14 by LPCVD.

Thereafter the doped polysilicon film 107 is selectively removed(patterned) by photolithography and etching, as shown in FIG. 11M. Thus,a select gate 15 is formed.

After the formation of the select gate 15, a silicon nitride film isformed on the third insulating film 14 by LPCVD. The silicon nitridefilm is formed in a thickness for burying the select gate 15 therein.The silicon nitride film is left only around the select gate 15 byetch-back to form a sidewall 16, as shown in FIG. 11N.

Thereafter an N-type impurity (arsenic ions, for example) ision-implanted into the whole area of an active region of thesemiconductor layer 2 from the surface thereof. A heat treatment isperformed for diffusing the N-type impurity, thereby forming a firstdiffusion region 3, a second diffusion region 6 and a third diffusionregion 7 on the surface layer portion of the semiconductor layer 2, asshown in FIG. 11O. Thus, the memory cell of the semiconductor device 91is obtained.

FIG. 12 is a schematic sectional view of a semiconductor deviceaccording to a sixth embodiment of the present invention. Referring toFIG. 12, portions corresponding to those shown in FIG. 2 are denoted bythe same reference numerals respectively. In relation to the structureshown in FIG. 12, the following description is made with reference topoints different from those of the structure shown in FIG. 2, andredundant description is omitted as to the portions corresponding tothose shown in FIG. 2.

In a semiconductor device 111 shown in FIG. 12, a first insulating film8 integrally includes a thin portion 112 in contact with a firstdiffusion region 3 and a second diffusion region 6, a first thickportion 113 formed on side surfaces of an opening-side end portion of atrench 5 to be continuous with the thin portion 112 and a second thickportion 114 formed on a side opposite to the first thick portion 113through the thin portion 112 to be continuous with the thin portion 112.The first thick portion 113 and the second thick portion 114 havethicknesses larger than that of the thin portion 112.

According to the structure, the first thick portion 113 is formed on theside surfaces of the opening-side end portion of the trench 5, wherebythe thin portion 112 and the first thick portion 113 are in contact withthe second diffusion region 6 (a drain region of a memory transistor17). Thus, the size of the thin portion 112 causing FN tunneling isreduced, whereby undesired charge escape from a floating gate 11 can besuppressed. Further, a large size can be ensured for a portion (aportion obtained by combining the first thick portion 113 and the secondthick portion 114 with each other) having a thickness larger than thatof the thin portion 112, and a coupling ratio can be further improved.

When the hydrofluoric acid is continuously supplied after the oxide film35 is removed from the portion of the semiconductor layer 2 locatedoutside the trench 5 in the step shown in FIG. 3K so that the oxide film35 is removed also from the side surfaces of the opening-side endportion of the trench 5, the first thick portion 113 can bespontaneously formed by thermal oxidation in the step shown in FIG. 3L.

While some embodiments of the present invention have been described, thepresent invention may be embodied in other ways. For example, while theEEPROM is employed in each of the aforementioned embodiments, thepresent invention can be applied to a structure including a floatinggate nonvolatile storage element such as a flash memory, an EPROM(Erasable Programmable Read Only Memory) or a DRAM (Dynamic RandomAccess Memory) other than the EEPROM.

The conductivity types of the semiconductor portions of thesemiconductor devices 1, 41, 71, 91 and 111 may be reversed. In otherwords, the P-type portions may be replaced with N-type portions and viceversa in the semiconductor devices 1, 41, 71, 91 and 111.

While the present invention has been described in detail by way of theembodiments thereof, it should be understood that these embodiments aremerely illustrative of the technical principles of the present inventionbut not limitative of the invention. The spirit and scope of the presentinvention are to be limited only by the appended claims.

This application corresponds to Japanese Patent Application No.2008-135514 filed with the Japan Patent Office on May 23, 2008, thedisclosure of this application is incorporated herein by reference.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor layer having a surface; a trench extending downward fromthe surface of the semiconductor layer, the trench having a first side,a second side opposite the first side, and a bottom; a source regionformed on a surface layer portion of the semiconductor layer adjacentlyto the first side of the trench in a prescribed direction; a drainregion formed on the surface layer portion of the semiconductor layeradjacently to the second side of the trench in the prescribed direction;a first insulating film formed on the bottom surface and the first andsecond side surfaces of the trench, the first insulating film having athin portion and having a thick portion formed by the remainder of thefirst insulating film, the thick portion having a thickness that isgreater than the thickness of the thin portion; a floating gate stackedon the first insulating film and opposed to the bottom surface and theside surfaces of the trench through the first insulating film; a secondinsulating film formed on the floating gate; and a control gate at leastpartially embedded in the trench so that the portion embedded in thetrench is opposed to the floating gate through the second insulatingfilm, wherein the thick portion of the first insulating film covers thebottom surface of the trench entirely.
 2. The semiconductor deviceaccording to claim 1, wherein the thin portion is in contact with anedge of the trench at the surface of the semiconductor substrate.
 3. Thesemiconductor device according to claim 1, wherein the thin portion isin contact with a region of the first side surface of the trenchexposing the drain region so as to be in contact with the drain region.4. The semiconductor device according to claim 1, wherein the thickportion has a first thick portion that is formed on the first sidesurface of an opening-side end portion of the trench and that iscontinuous with the thin portion, and also has a second thick portionthat is formed on the second side surface of the trench and that iscontinuous with the thin portion.
 5. The semiconductor device accordingto claim 1, wherein the floating gate, the second insulating film andthe control gate protrude beyond the upper end of the trench.
 6. Thesemiconductor device according to claim 5, further comprising a thirdinsulating film laminated on the semiconductor layer, wherein portionsof the floating gate, the second insulating film and the control gateprotruding beyond the upper end of the trench are covered with the thirdinsulating film.
 7. The semiconductor device according to claim 1,wherein the control gate integrally has a body portion protruding fromthe trench and an extending portion extending sideward from the bodyportion.
 8. The semiconductor device according to claim 7, furthercomprising a third insulating film laminated on the semiconductor layer,wherein an upper end portion of the second insulating film extends overthe floating gate and the third insulating film, and the extendingportion of the control gate is arranged on the second insulating filmand over the third insulating film.
 9. The semiconductor deviceaccording to claim 1, wherein a portion of the thin portion that isbetween the drain region and the floating gate and is in contact withthe side surface of the trench defines a tunnel window.
 10. Asemiconductor device comprising: a semiconductor layer having a surface;a trench extending downward from the surface of the semiconductor layer,the trench having a first side, a second side opposite the first side,and a bottom; a source region formed on a surface layer portion of thesemiconductor layer adjacently to the first side of the trench in aprescribed direction; a drain region formed on the surface layer portionof the semiconductor layer adjacently to the second side of the trenchin the prescribed direction; a first insulating film formed on thebottom surface and the first and second side surfaces of the trench, thefirst insulating film having a thin portion and having a thick portionformed by the remainder of the first insulating film, the thick portionhaving a thickness that is greater than the thickness of the thinportion; a floating gate stacked on the first insulating film andopposed to the bottom surface and the side surfaces of the trenchthrough the first insulating film; a second insulating film formed onthe floating gate; a control gate at least partially embedded in thetrench so that the portion embedded in the trench is opposed to thefloating gate through the second insulating film; and a third insulatingfilm laminated on the semiconductor layer, wherein the floating gate,the second insulating film and the control gate have upper ends that areflush with the surface of the third insulating film.